The only gothcha I have seen I have seen in this deployment scenario is: " Cascading contexts requires that you configure unique MAC addresses for each context interface. Because of the limitations of classifying packets on shared interfaces without MAC addresses, we do not recommend using cascading contexts without unique MAC addresses."

Also the not supported - VPN and dynamic routing protocols.

I have seen 3 Contexts working on a ASA 5520 with no problems in a 30MB Inet connection.